Should a finished gypsum product be exposed to relatively high temperatures, such as those produced by high temperature flames or gases, portions of the gypsum may absorb sufficient heat to start the release of water from the gypsum dihydrate crystals of the core. The absorption of heat and release of water from the gypsum dihydrate may be sufficient to retard heat transmission through or within the gypsum product for a time. The gypsum product can act as a barrier to prevent high temperature from passing directly therethrough. The heat absorbed by the gypsum product can be sufficient to essentially recalcine portions of the gypsum, depending on the heat source temperatures and exposure time. At certain temperature levels, the heat applied to a gypsum product also may cause phase changes to the anhydrite of the gypsum and rearrangement of the crystalline structures. In some instances, the presence of salts and impurities may affect the phase transition temperatures, resulting in a difference in crystal morphologies.
Gypsum panels have been produced that resist the effects of relatively high temperatures for a period of time, which may inherently delay passage of high heat levels through or between the panels, and into (or through) systems using them. Gypsum panels referred to as fire resistant or “fire rated” typically are formulated to enhance the panels' ability to delay the passage of heat though wall or ceiling structures and play an important role in controlling the spread of fire within buildings. As a result, building code authorities and other concerned public and private entities typically set stringent standards for the fire resistance performance of fire rated gypsum panels.
The ability of gypsum panels to resist fire and the associated extreme heat may be evaluated by carrying out generally-accepted tests. Examples of such tests are routinely used in the construction industry, such as those published by Underwriters Laboratories (“UL”), such as the UL U305, U419 and U423 test procedures and protocols, as well as procedures described in the specifications E119, e.g., E119-09a, published by the American Society for Testing and Materials (ASTM). Some of such tests comprise constructing test assemblies using gypsum panels, normally a single-layer application of the panels on each face of a wall frame formed by wood or steel studs. Depending on the test, the assembly may or may not be subjected to load forces. The face of one side of the assembly, such as an assembly constructed according to UL U305, U419 and U423, for example, is exposed to increasing temperatures for a period of time in accordance with a heating curve, such as those discussed in the ASTM E119 procedures.
The temperatures proximate the heated side and the temperatures at the surface of the unheated side of the assembly are monitored during the tests to evaluate the temperatures experienced by the exposed gypsum panels and the heat transmitted through the assembly to the unexposed panels. The tests are terminated upon one or more structural failures of the panels and/or when the temperatures on the unexposed side of the assembly exceed a predetermined threshold. Typically, these threshold temperatures are based on the maximum temperature at any one of such sensors and/or the average of the temperature sensors on the unheated side of the assembly.
Test procedures, such as those set forth in UL U305, U419 and U423 and ASTM E119, are directed to an assembly's resistance to the transmission of heat through the assembly as a whole. The tests also provide, in one aspect, a measure of the resistance of the gypsum panels used in the assembly to shrinkage in the x-y direction (width and length) as the assembly is subjected to high temperature heating. Such tests also provide a measure of the panels' resistance to losses in structural integrity that result in opening gaps or spaces between panels in a wall assembly, with the resulting passage of high temperatures into the interior cavity of the assembly. In another aspect, the tests provide a measure of the gypsum panels' ability to resist the transmission of heat through the panels and the assembly. It is believed that such tests reflect the specified system's capability for providing building occupants and firemen/fire control systems a window of opportunity to address or escape fire conditions.
In the past, various strategies were employed to improve the fire resistance of fire rated gypsum panels. For example, thicker, denser panel cores have been provided which use more gypsum relative to less dense gypsum panels, and therefore include an increased amount of water chemically bound within the gypsum (calcium sulfate dihydrate), to act as a heat sink, to reduce panel shrinkage, and to increase the structural stability and strength of the panels. Alternatively, various ingredients including glass fiber and other fibers have been incorporated into the gypsum core to enhance the gypsum panel's fire resistance by increasing the core's tensile strength and by distributing shrinkage stresses throughout the core matrix. Similarly, amounts of certain clays, such as those of less than about one micrometer size, and colloidal silica or alumina additives, such as those of less than one micrometer size, have been used in the past to provide increased fire resistance (and high temperature shrink resistance) in a gypsum panel core. It has been recognized, however, that reducing the weight and/or density of the core of gypsum panels by reducing the amount of gypsum in the core will adversely affect the structural integrity of the panels and their resistance to fire and high heat conditions.
Another approach has been to add unexpanded vermiculite (also referred to as vermiculite ore) and mineral or glass fibers into the core of gypsum panels. In such approaches, the vermiculite is expected to expand under heated conditions to compensate for the shrinkage of the gypsum components of the core. The mineral/glass fibers were believed to hold portions of the gypsum matrix together. There is a continuing need to develop gypsum products, e.g., at lower weight, that are less susceptible to the damaging effects of extreme heat.
It will be appreciated that this background description has been created to aid the reader, and is not to be taken as a reference to prior art nor as an indication that any of the indicated problems were themselves appreciated in the art.